Solaris 2.x releases are essentially frozen several months before their general release date. During the early access/beta test period bugs are found both in the beta and in the previous release. That's why the moment a new release comes out a number of patches is ready. Some of those are on the Solaris 2.x CD. Others were released almost at the same time as 2.x, or even before 2.x becomes generally available.
Some bugs are found in the previous release that can't be fixed in the time for the next release. And bugs get added to new code.